Energy may be harvested from the movement of body joints of humans and other animals by converting mechanical energy derived from such movement to electrical energy. Activities where body joints move repeatedly, such as walking, jogging, and running, for example, present opportunities to harvest energy from moving body joints over an extended period of time. In some energy harvesting devices and methods, a generator driven by joint motion is coupled to an electrical load. Since the instantaneous mechanical power provided by body joints during repetitive or cyclical activities typically varies over the period of each cycle, both the harvested electrical power supplied to the load and the associated forces applied to the body joint may be time-varying over each cycle.
Muscles may be described as working in a positive mechanical power mode or a negative mechanical power mode. A positive mechanical power mode exists when the product of angular velocity and torque about the associated body joint is positive, i.e. the knee joint is extending and pushing against the external force of gravity on the body away from the ground, or flexing and pulling the foot off the ground against the external force of gravity. A negative mechanical power mode exists when the product of angular velocity and torque about the associated body joint is negative, i.e. the knee joint is extending but resisting that extension, or flexing but resisting that flexion. Generator torque developed by a harvesting generator always counteracts or opposes motion of a body segment. The generator torque acts against muscles operating on a body segment that is moving in a positive mechanical power mode, thereby increasing the work that must be done by the muscles to move body segment. Harvesting energy from the movement of a body segment when associated muscles are operating in a positive mechanical power mode may be referred to as “non-mutualistic” energy harvesting, since the generator torque associated with such energy harvesting acts against muscles and generally increases the metabolic cost of the associated body segment motion. In contrast, generator torque aids muscles operating on a body segment that is moving in a negative mechanical power mode. Harvesting energy from the movement of a body segment when muscles associated with a body segment are operating in a negative mechanical power mode may be referred to as “mutualistic” energy harvesting, since it aids muscles and generally reduces the metabolic cost of the associated body segment motion.
Some energy harvesters are configured to preferentially harvest energy mutualistically by synchronizing energy harvesting to negative power modes of body segment. In some such harvesters, control logic achieves such synchronization based on one or more sensed characteristics of the motion of the host to which the body segment belongs. For example, control logic may synchronize energy harvesting to particular gait phase ranges, which it determines based on one or more sensed characteristics of the motion of the host to which body segment belongs.
FIG. 1 includes plots that are representative of various quantities relating to typical dynamics of a knee joint during a walking gait cycle 1. In graph A, plot 2 represents the angular velocity of the knee joint (i.e. the time derivative of the angle of the knee joint), where positive angular velocity represents movement in the knee extension direction and negative angular velocity represents movement in the knee flexion direction. In graph B, plot 3 represents the moment of the knee joint, where a positive moment represents torque in the extension direction and a negative moment represents torque in the flexion direction. In graph C, plot 4 represents the mechanical power associated with movement of the knee joint, where positive mechanical power represents power that increases the mechanical energy of the knee. Mechanical power (plot 4) represents the product of the torque (plot 3) and the angular velocity (plot 2) of the knee joint. The integral of the mechanical power (plot 4) represents the mechanical work performed by the knee joint. Beyond the knee joint, the total mechanical work expended during walking includes work performed by other parts of the body, such as the ankles, the toes, the hips and the arms.
Referring to FIG. 1, gait cycle 1 may generally be divided into a swing portion 6 and a stance portion 7. During the swing portion 6, the foot corresponding to the shaded knee (i.e. the right knee) is off of the ground. In the stance portion 7, the foot corresponding to the shaded knee is on the ground. Swing portion 6 may be further divided into a swing flexion portion 6A, during which the knee is flexing, and a swing extension portion 6B, during which the knee is extending. Stance portion 7 may be further divided into a stance/collision flexion portion 7A, during which the knee is flexing, and a stance extension portion 7B, during which the knee is extending. During one gait cycle 1, angular velocity plot 2 comprises extrema 2A, 2B, 2C and 2D which occur, respectively, in swing flexion portion 6A, swing extension portion 6B, stance/collision flexion portion 7A and stance extension portion 7B. These extrema correspond to the end of acceleration of the knee joint.
In plot 4, muscles are acting to decrease the mechanical energy of the knee joint in negative power intervals 4A, 4B and 4C of power plot 4. In interval 4A, knee flexor muscles are acting against the extension that occurs during swing extension in order to arrest extension of the knee prior to heel strike. In interval 4B, knee extensor muscles are acting against the flexion that occurs during stance/collision flexion when the mass of the human is transferred to the foot shortly after heel strike. In interval 4C, knee extensor muscles are acting against the flexion that occurs during swing flexion in order to arrest flexion of the knee prior to the start of swing extension. The knee is working in a positive power mode in interval 8, as it is in intervals 8A, 8B and 8C.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.